1. Field of the Invention
The present invention relates to CO.sub.2 jet spray systems, and more particularly, to an optical sensor for use with CO.sub.2 jet spray nozzles employed in a CO.sub.2 jet spray system.
2. Description of Related Art
One means for detecting CO.sub.2 snow in jet sprays which has been used by the assignee of the present applcation comprises a thermocouple CO.sub.2 snow sensor. The disadvantages of the thermocouple sensor are its slow response time, which resulted in wasted cleaning time and wasted gas, its expensive instrumentation, and the fact that it only provided indirect detection of the CO.sub.2 snow plume. In addition, the thermocouple CO.sub.2 snow sensor cannot be immersed in the CO.sub.2 cleaning plume, since it disturbs the spray characteristic of the plume.
A particle counter has heretofore been used to detect CO.sub.2 snow in jet spray systems built by the assignee of the present invention. However, the error margin using these devices is relatively great, the measurements are indirect, the equipment is expensive, and it is difficult to interface the counter to a robotic controller.
Aside from the above-discussed devices, there are no other CO.sub.2 snow sensors that are commercially available. A variety of light-based particle counting devices exist which might be adapted for use in a limited sense to detect solid CO.sub.2 snow. These devices include particle scatter detectors, Doppler anemometers, zone sensors, and obscuration-type sensors.
Scatter-type sensors are excellent for measuring airborne particles in a gas stream, or clean room environment, but have difficulty handling harsh temperature extremes induced by the CO.sub.2 cooling effect. In addition, scatter-type sensors frequently misdiagnose ice pellets resulting from the cooled CO.sub.2 particles. Doppler anemometers may be used to give simultaneous size and velocity measurements of particles (including CO.sub.2 particles) in a gas stream, but for the vast majority of applications, they are extremely price prohibitive. Zone sensing has two disadvantages relating to CO.sub.2 particle counting. First, zone sensing is not a real time procedure, and second, it is cost prohibitive. Detection of particles using beam obscuration is conducted in several off-the-shelf particle counters. These counters are relatively expensive, and suffer the same pitfalls as light scattering detectors concerning CO.sub.2 cooling and ice particle counting.
A trained operator can distinguish between snow that has good cleaning ability. However, in an automated system, operator interaction should be eliminated because it is slightly subjective, and gives rise to significant errors. Various checks and safety devices are typically built into conventional robotic CO.sub.2 snow systems. However, a conventional robotic system may perform a complete cleaning cycle without any CO.sub.2 gas escaping from the nozzles. This condition is not easily detected in conventional systems. After opening of the jet spray valve, there is always some lead time before productive snow emerges. Waiting a set amount of time before start of the cleaning cycle is inefficient in time and CO.sub.2 management. At a point when liquid CO.sub.2 becomes depleted, sufficient cleaning snow is no longer produced. However, high pressure gas still sprays out of the nozzle and gives the appearance of snow. Detecting this condition can be difficult for even a trained operator.
Therefore, it is an objective of the present invention to provide for an optical sensor for use with CO.sub.2 jet spray nozzles employed in CO.sub.2 jet spray systems.